1. Field of the Invention
The present invention relates to charge transfer devices and, more particularly, is directed to a charge transfer device for use in a CCD (charge-coupled device) image sensor, a CCD linear sensor or the like.
2. Description of the Prior Art
As a charge transfer device in which a charge transfer portion is formed of a CCD, there are known a CCD image sensor, a CCD linear sensor, a CCD delay line or the like.
A CCD image sensor of an interline type comprises, as shown in FIG. 1 of the accompanying drawings, light sensing sections 21, each of which is formed of a photo-diode, arrayed in the horizontal and vertical directions in a matrix fashion, a vertical register 22 commonly provided in association with the light sensing sections 21 on the common vertical line, and a horizontal register 23 commonly provided for the respective vertical registers 22.
A signal charge accumulated in the light sensing section 21 during a charge storage period is read out to the vertical register 22. Then, during the horizontal blanking period, a signal charge is transferred at every row and a signal charge accumulated in the final stage of the vertical register 22 is transferred to the horizontal register 23. In the next horizontal output period (corresponding to one horizontal scanning period of television signal), a signal charge on the horizontal register 23 is transferred to an output section 24 side and is thereby output from the output section 24 as an imager signal S.
In particular, the vertical register 22 includes transfer stages, one set of which is formed of, for example, four transfer gate electrodes, arrayed in the vertical direction. Drive pulses having different phases are applied to the respective transfer gate electrodes, whereby the signal charge read out to the vertical register 22 from the light sensing section 21 is transferred to the horizontal register 23 side.
Although respective transfer gate electrodes are formed on the vertical register 22 by utilizing a conductor forming region provided between the light sensing sections 21, the four transfer gate electrodes cannot be formed of exactly the same pattern. As a result, from a conductor forming standpoint, a contact area of each transfer electrode on the vertical register 22 is fluctuated unavoidably.
As shown in FIG. 2, when transfer gate electrodes G1 and G3 to which drive pulses V1, V3 are applied are formed to be wider than transfer gate electrodes G2 and G4 to which drive pulses V2, V4 are applied so as to have a large contact area with the vertical register 22, added capacitances of the transfer gate electrodes G1, G3 become larger than those of the transfer gate electrodes G2, G4 so that waveforms of the drive pulses V1, V3 are blunted as compared with those of the drive pulses V2, V4.
Accordingly, as shown in FIG. 3, before the drive pulse V1 that is applied to the transfer gate electrode G1 goes to low (L) level fully, the drive pulse V4 that is applied to the adjacent transfer gate electrode G4 goes to high (H) level at timing point t.sub.1. At the next timing point t.sub.2, before the drive pulse V2 that is applied to the transfer gate electrode G2 goes to low level fully, the drive pulse V1 that is applied to the adjacent transfer gate electrode G1 goes to high level. Consequently, a maximum amount of electric charges handled by the vertical register 22 is decreased. There is then the disadvantage that characteristics of the CCD image sensor are deteriorated.